1. Field of Art
The disclosure relates generally to earth boring bits used to drill a borehole for applications including the recovery of oil, gas or minerals, mining, blast holes, water wells and construction projects. More particularly, the disclosure relates to percussion hammer drill bits. Still more particularly, the disclosure relates to percussion hammer drill bits with adjustable chokes.
2. Background of Related Art
In percussion or hammer drilling operations, a drill bit mounted to the lower end of a drill string simultaneously rotates and impacts the earth in a cyclic fashion to crush, break, and loosen formation material. In such operations, the mechanism for penetrating the earthen formation is of an impacting nature, rather than shearing. The impacting and rotating hammer bit engages the earthen formation and proceeds to form a borehole along a predetermined path toward a target zone. The borehole created will have a diameter generally equal to the diameter or “gage” of the drill bit.
A typical percussion drilling assembly is connected to the lower end of a rotatable drill string and includes a downhole piston-cylinder assembly coupled to the hammer bit. The impact force is generated by the downhole piston-cylinder assembly and transferred to the hammer bit via a driver sub. During drilling operations, a pressurized or compressed fluid (e.g., compressed air) flows down the drill string to the percussion drilling assembly. A choke is provided to regulate the flow of the compressed fluid to the piston-cylinder assembly and the hammer bit. A fraction of the compressed fluid flows through a series of ports and passages to the piston-cylinder assembly, thereby actuating the reciprocal motion of the piston, and then is exhausted through a series of passages in the hammer bit body to the bit face. The remaining portion of the compressed fluid flows through the choke and into the series of passages in the hammer bit body to the bit face. The compressed fluid exiting the bit face serves to flush cuttings away from the bit face to the surface through the annulus between the drill string and the borehole sidewall.
To promote efficient penetration by the hammer bit, the bit is “indexed” to fresh earthen formations for each subsequent impact. Indexing is achieved by rotating the hammer bit a slight amount between each impact of the bit with the earth. The simultaneous rotation and impacting of the hammer bit is accomplished by rotating the drill string and incorporating longitudinal splines which key the hammer bit body to a cylindrical sleeve (commonly known as the driver sub or chuck) at the bottom of the percussion drilling assembly. The hammer bit is rotated through engagement of a series of splines on the bit and driver sub that allow axial sliding between the components but do not allow significant rotational displacement between the hammer assembly and bit. As a result, the drill string rotation is transferred to the hammer bit itself. Rotary motion of the drill string may be powered by a rotary table typically mounted on the rig platform or top drive head mounted on the derrick.
Without indexing, the cutting structure extending from the lower face of the hammer bit may have a tendency to undesirably impact the same portion of the earth as the previous impact. Experience has demonstrated that for an eight inch hammer bit, a rotational speed of approximately 20 rpm and an impact frequency of 1600 bpm (beats per minute) typically result in relatively efficient drilling operations. This rotational speed translates to an angular displacement of approximately 5 to 10 degrees per impact of the bit against the rock formation.
The hammer bit body may be generally described as cylindrical in shape and includes a radially outer skirt surface aligned with or slightly recessed from the borehole sidewall and a bottomhole facing cutting face. The earth disintegrating action of the hammer bit is enhanced by providing a plurality of cutting elements that extend from the cutting face of the bit for engaging and breaking up the formation. The cutting elements are typically inserts formed of a superhard or ultrahard material, such as polycrystalline diamond (PCD) coated tungsten carbide and sintered tungsten carbide, that are press fit into undersized apertures in bit face. During drilling operations with the hammer bit, the borehole is formed as the impact and indexing of the drill bit, and thus cutting elements, break off chips of formation material which are continuously cleared from the bit path by pressurized air pumped downwardly through ports in the face of the bit.
In oil and gas drilling, the cost of drilling a borehole is very high, and is proportional to the length of time it takes to drill to the desired depth and location. The time required to drill the well, in turn, is greatly affected by the number of times the drill bit must be changed before reaching the targeted formation. This is the case because each time the bit is changed, the entire string of drill pipe, which may be miles long, must be retrieved from the borehole, section by section. Once the drill string has been retrieved and the new bit installed, the bit must be lowered to the bottom of the borehole on the drill string, which again must be constructed section by section. As is thus obvious, this process, known as a “trip” of the drill string, requires considerable time, effort and expense. Accordingly, it is always desirable to employ drill bits which will drill faster and longer, and which are usable over a wider range of formation hardness.
The length of time that a drill bit may be employed before it must be changed depends upon its rate of penetration (“ROP”), as well as its durability. The form and positioning of the cutting elements upon the bit face greatly impact hammer bit durability and ROP, and thus are critical to the success of a particular bit design.
For some conventional percussion drilling assemblies, drilling efficiency and ROP decreases with drilling depth. In particular, as drilling depth increase, backpressure in the annulus that acts against the bit face increases, thereby reducing the effective force with which the hammer bit impacts the fresh formation. One conventional means to counteract the detrimental effects of increased backpressure is to increase the volume and/or pressure of the compressed fluid flowed through the percussion drilling assembly at the surface. However, in many operations, the ability to increase the volume and/or pressure of the compressed fluid is limited by the capacity of the compressors at the surface. Once the maximum capacity of the compressors is attained, additional backpressure increases detrimentally affect cutting efficiency and ROP.
In addition, while drilling through a payzone or lower pressure reservoir, it is typical for the operator to switch the drilling fluid from compressed air to nitrogen. This typically depends, at least in part, on the type and concentration of the hydrocarbon. The change to nitrogen drilling fluid primarily serves to reduce the potential for a downhole fire, which would occur in the presence of compressed air containing as much as 20% oxygen. In most cases, oxygen concentrations of 5-10% are required to stay below the flammability limit. The use of nitrogen generating units has been established as a safe and economical means of generating nitrogen to facilitate gas drilling in formations producing hydrocarbons. However, these units typically operate on the principle of membrane filtration, which limits the throughput to 50-70% depending on the level of filtration desired. As an example, a 8¾ inch diameter hammer bit using approximately 3,000 scfm of air will only have approximately 1,500 to 2,100 scfm after the changeover to nitrogen, all other factors being constant. Although it is common to have additional compressors on location to be brought on-line when the changeover occurs, it adds to significantly to the overall costs of the drilling operation.
Using the same example above, the hammer may have a choke installed, typically a ¼″ diameter orifice. This choke bypasses a fraction of the compressed air on the order of a few hundred scfm. When the switchover from compressed air to nitrogen is made, the reduced volume available will lower the driving pressure and thereby result in a lower energy delivered by the hammer bit. The presence of a choke further compounds the problem, in that, even at the reduced volume available, a fraction of the volume continues to be bypassed through the choke, reducing the driving pressure even further.
Accordingly, there is a need for percussion drilling assemblies and hammer bits that offer the potential to maintain drilling efficiency and ROP under increased annulus backpressures and/or with changes in the compressed fluid. Such improved hydraulics would be particularly well received if they were adjustable during downhole drilling operations (i.e., without requiring a trip of the drill string).